1. Field of the Invention
The present invention relates to a wireless communication technology implemented in a mesh network.
2. Description of the Related Art
In recent years, the use of a mesh network for performing wireless communication is being extensively studied. The mesh network is one of the network forms that include a star network, a bus network, a ring network, and the like. More particularly, the mesh network is a reticulate network in which each of a plurality of communication devices can mutually communicate with all other communication devices via a plurality of communication routes. Thus, even if a problem occurs in one of the communication routes, the communication can be performed via the rest of the communication routes. Such a feature facilitates in configuring a failure-resistant mesh network. Moreover, to maintain the communication speed in a mesh network, a communication route can be selected based on the data communication traffic. A common example of a mesh network is a power distribution network or the Internet network.
In a mesh network used for a wireless communication (hereinafter, “wireless mesh network”), direct communication of wireless signals between two communication devices is possible if the communication devices are arranged in a direct communicable range. However, even if the communication devices are arranged out of the direct communicable range, the wireless signals can be communicated by arranging one or more intermediate communication devices therebetween. Furthermore, because large amount of transmission power is necessary to perform direct communication between two distant communication devices, the communication performed via the intermediate communication devices is useful from an energy saving perspective. Meanwhile, such a wireless mesh network can be easily incorporated with a wired network by connecting any one of the communication devices to the wired network. That eliminates the need of a conventional configuration in which an access point, which is connected to the wired network, needs to be installed at the center of the communicable range of a wireless network. Instead, by using a communication device that is arranged near an end limit of the communicable range as the access point for connection with the wired network, the cabling can be restricted to the minimum.
Moreover, it is also possible to easily expand the communicable range of the wireless mesh network. Conventionally, when an access point is installed at the center of the communicable range, the communicable range can be expanded by enhancing the output capacity from the access point or by installing additional access points. Generally, to enhance the output capacity from an access point, it is necessary to replace the device thereat. Similarly, while installing additional access points, it is necessary to lay the communication line up to the additional access points. On the other hand, in the wireless mesh network, the communicable range can be easily expanded by newly arranging a communication device at the desired place for communication outside the current communicable range and supplying power to the newly arranged communication device.
In such a wireless mesh network, a significant factor while performing communication is the selection of a suitable communication route from among a plurality of communication routes. For example, Japanese Patent Application Laid-Open No. 2006-186565 discloses a mesh network in which a separate communication route is allotted for transmission of each signal to prevent overlapping of communication routes. More particularly, consider a case when an intermediate communication device is transmitting a first signal and receives a request for transmission of a second signal, which has a higher priority than the first signal. Then, according to Japanese Patent Application Laid-Open No. 2006-186565, the intermediate communication device transmits the second signal and informs a source communication device of the first signal about the subsequent communication route for the second signal. Based on that information, the first signal is transmitted via another communication route and overlapping of the communication routes is prevented. Such a configuration reduces the throughput of the mesh network in entirety.
Generally, a communication route is selected based on the number of intermediate communication devices arranged therein. The number of intermediate communication devices corresponds to the number of hops taken while transmitting a wireless signal via a communication route. Because an overhead time is generated when each intermediate communication device in a communication route transmits a wireless signal, it is desirable that a communication route having the least number of intermediate communication devices is selected such that the transmission is completed at the earliest. Moreover, the actual time for transmission of a wireless signal from one communication device to another communication device depends on the distance therebetween. By taking into consideration the transmission speed of a wireless signal, the overhead time generated at each intermediate communication device affects the actual time for transmission more than the distance between the source communication device and the destination communication device does.
However, as the distance between two intermediate communication devices in a communication route increases, the transmitted wireless signal gets weaker thereby reducing the reception power of the communication device that receives the wireless signal. Moreover, there is also a possibility that the noise increases during the transmission. Thus, longer the distance between the communication devices, more is the possibility that the signal quality degrades. Consequentially, a signal to noise ratio (SNR) at the communication device that receives the signal also degrades. If an error rate for the wireless signal is at a correctable level, then the actual time for transmission is not substantially affected. However, when the error rate reaches an uncorrectable level, then it becomes necessary to retransmit the signal thereby increasing the actual time for transmission. Thus, to sum up, selecting a communication route having the least number of intermediate communication devices does not guarantee speedy communication of a wireless signal.